The present invention relates to devices that communicate over a wireless mesh network using a Time Division Multiple Access (TDMA) communication protocol. In particular, the present invention relates to a wireless mesh network using multisized timeslots for communication among devices having different communication speed capabilities.
Wireless data communication and control will be a dominant player in future sensor automation, process control, security, and safety regulation. One of the important requirements for wireless data communication and control is minimized power consumption by the devices communicating over the network.
In wireless mesh network systems designed for low power, sensor/actuator-based applications, many devices in the network must be powered by long-life batteries or by low power energy-scavenging power sources. Power outlets, such as 120VAC utilities, are typically not located nearby or may not be allowed into the hazardous areas where the instrumentation (sensors) and actuators must be located without incurring great installation expense. The need for low installation cost drives the need for battery-powered devices communicating as part of a wireless mesh network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a well functioning wireless device. Batteries are expected to last more than 5 years and preferably as long as the life of the product.
In a true wireless mesh network, which may also be referred to as a self-organizing multi-hop network, each device (or node) must be capable of routing messages for itself as well as other devices in the network. The concept of messages hopping from node to node through the network is beneficial because lower power RF radios can be used, and yet the mesh network can span a significant physical area delivering messages from one end to the other. High power radios are not needed in a mesh network, in contrast a point-to-point system, which employs remote devices talking directly to a centralized base-station.
A mesh network protocol allows for the formation of alternate paths for messaging between devices and messaging between the devices and a data collector, bridge or gateway to some higher level higher-speed data bus. Having alternate, redundant paths for wireless messages enhances data reliability by ensuring there is at least one alternate path for messages to flow even if another path is blocked or degrades due to environmental influences or interference.
Some mesh network protocols are deterministically routed such that every device has an assigned parent and at least one alternate parent. In the hierarchy of the mesh network, much as in a human family, parents have children, children have grandchildren, and so on. Each device (or node) relays the messages for its descendants through the network to some final destination such as a gateway. The parenting nodes may be battery-powered or limited-energy powered devices. The more descendants a node has, the more traffic it must route, which in turn directly increases its own power consumption and diminishes its battery life.
In order to save power, some protocols limit the amount of traffic any node can handle during any period of time by only turning on the radios of the nodes for limited amounts of time to listen for messages. Thus, to reduce average power, the protocol may allow duty-cycling of the radios between On and Off states. Some protocols use a global duty cycle to save power such that the entire network is On and Off at the same time. Other protocols (e.g. TDMA-based) use a local duty cycle where only the communicating pair of nodes that are linked together are scheduled to turn On and Off in a synchronized fashion at predetermined times. Typically, the link is pre-determined by assigning the pair of nodes a specific timeslot for communications, assigning an RF frequency channel to be used by the radios, and designating who is to be receiving (Rx), and who is to be transmitting (Tx) at that timeslot.
When a new device joins the network, a network manager provides the new device with a schedule which the new device will use to talk to other devices in the network. Each device in the network is provided with timeslots (specific times and radio frequencies) for passing data to or from one or more “children” and one or more “parents”. Using different times and frequencies allows many devices to pass messages in the same space without collisions. Frequency hopping also helps to secure the data that is being passed in the network. Secured self-organizing networks frequently employ authentication and encryption to further protect the network.
In a TDMA network, a timeslot represents a communication window. A series of timeslots make up a frame, which is a repeating unit of time that defines a refresh rate of the network.
A typical TDMA based wireless mesh network breaks the frame into equal duration timeslots. Each slot is then scheduled to support communication from one device to another. Timeslots are defined as the minimum amount of time needed to turn on the radio, verify the channel is clear (listen), send the message, and listen for an acknowledgement. Radios must switch between receive-transmit-receive during this process and this turnaround time is a factor in the minimum slot time, as is the packet size of the message being sent.
Several performance enhancements to wireless mesh networks will occur over time as silicon radios get better, e.g. faster turn around time, higher speeds of communication, compression of data, less clock drift, etc. All of these enhancements and many more will lead to smaller and smaller timeslots. Forward compatibility of wireless mesh networks will become an issue as new devices joining a network will have communication speed capabilities that are much greater than other devices in the network.